The invention lies in the field of welding technology and relates to a process and an apparatus for welding sheet metal edges, in particular for the manufacture of container bodies in the form of hollow cylinders by welding together (for example by resistance welding, mash welding or laser welding) the edges of a metal sheet which has been suitably cut to size and then subjected to a rounding process in which opposite edges of the sheet are juxtaposed, or for the manufacture of composite panels by welding together flat sheet metal parts, the said panels subsequently being shaped by deep drawing.
According to the state of the art, the bodies of sheet metal cans, for example cans made from sheet steel or tin plate, are produced by cutting pieces of sheet metal from a coil, rounding them in a rounding apparatus, and welding together the edges that has been juxtaposed by the rounding, usually by electrical resistance welding, or by laser welding or another welding process. To weld the edges, they are precisely positioned, for example by being guided in a Z-shaped rail with two guide slots, and are guided continuously through a welding station which in the case of resistance welding usually comprises two welding electrodes in the form of rollers between which the overlapping sheet metal edges to be welded are guided and simultaneously welded.
With a view to stepping up production for the welding of composite panels or can bodies including those with extended weld seams, whereby the product can be made more cheaply, there is a desire to speed up the welding of the sheet metal edges, that is to say, to guide them through the welding station more and more rapidly so that the material to be welded has to be heated to welding temperature more and more rapidly. The application of the necessary thermal energy limits the welding speed on the one hand because of purely physical considerations. For example, in the case of resistance welding with excessive power flows, when non-uniformities cannot be avoided, spatter is produced due to local over heating; and in the case of laser welding with excessive power flows, the laser is defocused in the resulting metal vapour plasma. On the other hand, the stresses occurring in the material when it is heated are greater if the heating rate is increased, and can adversely affect the quality of the product. As a result, the choice of materials which can be used is restricted, and in particular it is difficult or impossible to weld cheaper material of lower quality at equally high speeds, as the susceptibility to cracking due to thermal stresses is higher in such materials.
It is an object of the invention to provide a process and an apparatus by means of which the welding speed can be increased beyond the above-mentioned limits without thereby necessitating costly elaboration to the equipment that is required.
It should therefore be possible, with the process and the apparatus according to the invention, for example to weld sheet metal edges more rapidly in materials which can be welded without any problem according to the state of the art, and to weld can bodies or panels in materials of lower quality at the speeds which are usual according to the state of the art without any loss of quality.
This object can be achieved by the process and apparatus defined in the claims.